1. Field of the Invention
The invention relates to a voltage-to-frequency conversion circuit, and more particularly to a multi-input voltage-to-frequency conversion circuit.
2. Description of the Related Art
Referring to FIG. 1, a conventional voltage-to-frequency conversion circuit is shown to include an operational amplifier 11, a voltage-to-current converter 12 coupled to the operational amplifier 11, and a current-controlled oscillator 13 coupled to the voltage-to-current converter 12. The operational amplifier 11 receives an input voltage (Vin) at its inverting input end 111, and amplifies the input voltage (Vin) so as to output an amplified voltage (Vamp) at its output end. The voltage-to-current converter 12 receives the amplified voltage (Vamp) from the output end of the operational amplifier 11, and converts the amplified voltage (Vamp) into a sensed current (I1). The current-controlled oscillator 13 is operable to generate a periodic pulse signal based on the sensed current (I1), and outputs the periodic pulse signal at an output end 131 thereof. The periodic pulse signal has a frequency linearly proportional to the magnitude of the input voltage (Vin).
In such a configuration, only the inverting input end 111 of the operational amplifier 11 is used to receive the single input voltage (Vin) to be converted at each time. Therefore, if there are different input voltages to be converted, which correspond respectively to various biomedical signals associated with the same human or animal body and are provided respectively by different signal lines, the conventional voltage-to-frequency conversion circuit has to convert the different input voltages one by one. It is noted that, prior to each conversion, the inverting input end 111 of the operational amplifier 11 must be manually disconnected from a previous signal line, and then connected to another signal line for processing a desired input voltage. In this case, contact resistances each induced by electrical connection between the inverting input end 111 of the operational amplifier 11 and a corresponding signal line may differ from each other. In addition, it may not be ensured that periodic pulse signals obtained by the conventional voltage-to-frequency conversion circuit may correspond respectively to the different input voltages under the same physiological condition of the human or animal body. As a result, there may be an error in a biomedical signal-based integrated judgment made for the human or animal body based on the periodic pulse signals provided by such conventional voltage-to-frequency conversion circuit.